I used to have a plastic rectangular water tank that fit into my hitch basket. The only issue I had was finding a hitch basket that was big enough to fit a rectangular plastic tank full of water ( weight and dimensions need to be calculated)
Then I had to find a tank that would fit in the basket, so I searched for both items at the same time.
I had a fill cap on the top of the tank in the back right side just above the drain valve which was in the bottom right side corner.
I threaded on a SS valve with SS quick connect plus SS fittings for the pump to attach to.
I could quickly hook it up the 12 volt pump and add water to my camper storage tank, or use the same pump to fill the tanks from creeks.
These two links are NOT what I had, but gives you an idea.
Hitch cargo basketPlastic water tank something like this.Soup.